This proposal is significant because we aim to develop novel treatments for lymphedema, a common and morbid complication of cancer treatment that results in chronic swelling, recurrent infections, and immune dysfunction. Recent clinical studies have reported decreased swelling and symptomatic relief after lymph node transfer (LNT) to the affected limb. To study the mechanisms of this process, we have developed a mouse model of LNT and have shown that the lymphatic vessels of the lymph node spontaneously reconnect with the lymphatics of the recipient site thereby restoring lymphatic circulation. However, although these results are promising, it remains unclear if immunologic function is restored after LNT. This gap in our knowledge is important because understanding the mechanisms by which LNT regulates immune function and microenvironmental changes is a necessary step for improving these outcomes. Our proposed study will address this gap and will therefore break new ground in the treatment of lymphedema. Our approach is innovative because we have developed an inducible mouse model of lymphedema in which tissue lymphatics can be selectively ablated using diphtheria toxin. This enables us to study immune function after LNT in a clinically relevant model and test the central hypothesis that lymphatic regeneration after LNT restores immune function and promotes resolution of tumorigenic microenvironmental changes. Aim 1: Determine how LNT restores immune function. This aim will test the hypothesis that innate and adaptive immune function is restored after LNT. The rationale for these studies is based on the fact that patients who have undergone lymph node dissection or suffer from lymphedema have impaired antibody response and are at increased risk for infections. Aim 2: Determine how LNT regulates lymphatic injury induced microenvironmental changes and tumor recurrence. This aim will test the hypothesis that LNT decreases activation of tumorigenic microenvironmental changes known to occur after lymphatic injury. This hypothesis is based on the rationale that 1) Patients with breast cancer are at high risk for loco-regional recurrence; 2) Lymphatic injury increases expression of tumorigenic pathways including interleukin 6 as well as macrophages and Treg infiltration; and 3) Previous animal studies have shown that lymphatic injury increases the growth/metastasis of breast cancers. We will test our mouse model of LNT together with microscopic orthotopic and heterotopic breast tumor implants to determine how restoration of immune responses by LNT regulate growth of local recurrence or development of a new primary cancer. As a result of these studies, we expect to further our understanding of the mechanisms by which lymphatic injury regulates tumor growth/spread and how these changes are modulated by LNT. At the conclusion of the proposed study we expect to understand the mechanisms that regulate impaired immune function and promote tumorigenic environmental changes after lymphatic injury and how these responses are altered by LNT. These studies are therefore not only important not to patients with lymphedema but will also increase our general understanding of how the lymphatic system can regulate tumor growth.